A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays
are stopped in a 500 μm thick layer of silicon diodes, and collected charge is processed by an attached ASIC. Goals
of the project are high flux (108 x-rays/s/pixel) capability and fast readout (< 0.5 ms dead time between frames). "Mixed-Mode" refers to a readout method whereby integrated signal accumulating in each pixel is compared against a threshold
value. When the threshold is reached, a digital count is added to an 18-bit in-pixel counter and a set quantity of charge is
removed from integrator. At the end of the x-ray exposure, analog signal left in the integrator is separately processed.
Thus, one obtains mixed digital and analog data where the counter bits are a high order word and the analog residual
provides higher precision. Typically, each count is equivalent to 100 10 keV x-rays, for a well-depth >107 10 keV x-rays/
pixel. The analog residual is digitized to 9-bit precision allowing measurement of the residual charge to better than
a quarter of the charge from single 10 keV x-rays. Measurements are presented on x-ray tests at the Cornell High Energy
Synchrotron Source (CHESS). Dynamic range, linearity, point-spread function and noise properties are shown. Status
will be is reported on five different approaches for ASIC-diode hybridization. Progress toward bonding of a 128 x 512
pixel device is also presented.
KEYWORDS: Sensors, Signal to noise ratio, Charge-coupled devices, Electron microscopy, Photons, Active sensors, Modulation transfer functions, Spatial resolution, Cadmium sulfide, Electron beams
In a longstanding effort to overcome limits of film and the charge coupled device (CCD) systems in electron microscopy, we have developed a radiation-tolerant system that can withstand direct electron bombardment. A prototype Direct Detection Device (DDD) detector based on an Active Pixel Sensor (APS) has delivered unprecedented performance with an excellent signal-to-noise ratio (approximately 5/1 for a single incident electron in the range of 200-400 keV) and a very high spatial resolution. This intermediate size prototype features a 512×550 pixel format of 5&mgr;m pitch. The detector response to uniform beam illumination and to single electron hits is reported. Radiation tolerance with high-energy electron exposure is also impressive, especially with cooling to -15 °C. Stable performance has been demonstrated, even after a total dose of 3.3×106 electrons/pixel. The characteristics of this new detector have exciting implications for transmission electron microscopy, especially for cryo-EM as applied to biological macromolecules.
High resolution electron imaging is very important in nanotechnology and biotechnology fields. For example, Cryogenic Electron-Microscopy is a promising method to obtain 3-D structures of large protein complexes and viruses. We report on the design and measurements of a new CMOS direct-detection camera system for electron imaging. The
active pixel sensor array that we report on includes 512 by 550 pixels, each 5 by 5 μm in size, with an ~8 μm epitaxial layer to achieve an effective fill factor of 100%. Spatial resolution of 2.3 μm for a single incident e- has been measured. Electron microscope tests have been performed with 200 and 300 keV beams, and the first recorded Electron Microscope image is presented.
KEYWORDS: Sensors, Active sensors, Signal to noise ratio, Electron microscopy, Diodes, CCD image sensors, Detector arrays, CCD cameras, Charge-coupled devices, Electron beams
There is an urgent need to replace film and CCD cameras as recording instruments for transmission electron
microscopy (TEM). Film is too cumbersome to process and CCD cameras have low resolution, marginal to poor
signal-to-noise ratio for single electron detection and high spatial distortion. To find a replacement device, we have
tested a high sensitivity active pixel sensor (APS) array currently being developed for nuclear physics. The tests were
done at 120 keV in a JEOL 1200 electron microscope. At this energy, each electron produced on average a signal-tonoise
ratio about 20/1. The spatial resolution was also excellent with the full width at half maximum (FWHM) about
20 microns. Since it is very radiation tolerant and has almost no spatial distortion, the above tests showed that a high
sensitivity CMOS APS array holds great promise as a direct detection device for electron microscopy.
A 2D photon counting digital pixel array detector is being designed for static and time resolved protein crystallography. This room temperature detector will significantly enhance monochromatic and polychromatic protein crystallographic throughput data rates by more than two or three orders of magnitude when compared to present data collection systems. The detector has an unbounded photon counting dynamic range and exhibits superior spatial resolution when compared to present crystallographic phosphor imaging plates or phosphor coupled CCD detectors. The detector is a high resistivity N-type Si with a pixel pitch of (150 X 150) microns, and a thickness of 300 microns that is bump bonded to an application specific integrated circuit. The event driven readout of the detector is based on the column architecture and allows an independent pixel bit rate above 1 million photons/sec. The device provides energy discrimination and sparse data readout that yields minimal dead time. This type of architecture allows an almost continuous (frame-less) data acquisition, a feature not found in any current detector being used for protein crystallographic applications. For the targeted detector size of (1000 X 1000) pixels, average hit rates greater than 1011 photons/sec for the complete detector appears achievable. This paper will present an overview of the hybridized detector performance which includes the analog amplifier response and the photon counting capabilities of the (16 X 16) array operating with both digital and analog circuitry. Also the operation of the serial interface will be described.
A 2D pixel array image sensor module has been designed for time resolved Protein Crystallography. This smart pixels detector significantly enhances time resolved Laue Protein crystallography by two or three orders of magnitude compared to existing sensors like films or phosphor screens coupled to CCDs. The resolution in time and dynamic range of this type of detector will allow to study the evolution of structural changes that occur within the protein as a function of time. This detector will also considerably accelerate data collection in static Laue or monochromatic crystallography and make better use of the intense beam delivered by synchrotron light sources. The event driven pixel array detectors, based on the column architecture, can provide multiparameter information (energy discrimination, time), with sparse and frameless readout without significant dead time. The prototype module consists of a 16 by 16 pixel diode array bump-bonded to the integrated circuit. Different detector materials (Silicon, CdZnTe) are evaluated. The detection area is 150 by 150 micrometers2 connected to the readout electronics. The individual pixel processor consists of a low-noise amplifier shaper followed by a differential threshold comparator which provides the counting of individual photons with an energy above a programmable threshold. To accommodate the very high rates, above 5 by 108/cm2/s, each pixel processor has a 3 bit pre-scaler which divides the event rate by 8. Overflow from the divider which defines a pseudo fourth bit will generate a readout sequence providing the pixel address. Addresses, generated locally as analog signals, are converted off-chip and used to increment a location in an histogramming memory to generate the computerized image of the Laue diagram.
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